This application claims the priority of German application No. 100 36 620.1, filed Jul. 27, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process and an apparatus for chroming an inner surface, in particular a cavity, of a metallic component.
To save weight or for cooling purposes, metallic components, such as turbine blades of stationary gas turbines or aircraft engines, can be of hollow design and have a cavity with an inner surface. In the case of turbine blades, the inner surface has to be chromed, on account of the risk of corrosion or sulfuration caused by sulphur.
A powder pack process for chroming the inner surface of the cavity of turbine blades is known, in which a powder mixture comprising Al2O3, chromium and an activator, such as NH4Cl, is introduced into the cavity. When the powder mixture is heated while hydrogen is being supplied, chromium is deposited, so as to form a chromium-containing diffusion layer. The introduction and removal of the powder mixture into and from the cavity have proven to be drawbacks of this process. During introduction, the complete covering of the inner surface of the cavity which is required in order to form a continuous diffusion layer causes problems in particular with complicated geometry or sharp edges. After the process, it is difficult to remove the powder pack from the cavities without leaving any residues. Powder residues often adhere to the inner surface of the cavity.
The object of the present invention is to provide a process in which an inner surface which is to be coated does not have to be brought into contact with a powder which forms the coating gas. Furthermore, it is intended to provide an apparatus for chroming an inner surface of a component.
With regard to the process, the solution according to the present invention is characterized by the following steps:                (1) providing a mixture of chromium granules and an activator;        (2) heating the mixture to a temperature at which a substantially gaseous coating gas comprising CrCl is formed;        (3) dissipating the coating gas; and        (4) exposing the inner surface of the component to the coating gas so as to form a chromium-containing diffusion layer.        
The advantage of the process is that the coating of the inner surface of the cavity takes place in the gas phase. Therefore, the introduction of a powder mixture into the cavity at the start of the process and the removal of the powder pack after the coating has been carried out are eliminated. Moreover, it is impossible for any powder residues to adhere to the coated surface of the cavity. The mixture, which substantially comprises granules, e.g. with a particle size of 5-20 mm, can be processed more rapidly and more economically than a powder mixture comprising a donor powder and a filler powder for preventing sintering. The granules do not cause any blockages which could impede the dissipation of the coating gas. Moreover, the granules are broken down gradually and do not have to be exchanged after each coating process, as is the case with a powder.
The mixture can be prepared using approximately 99% by weight of chromium granules and approximately 1% by weight of activator, it being possible for the activator to be, for example, NH4Cl in powder form.
To form the coating gas, the mixture can be heated at a temperature of approximately 1200° C.
The dissipation of the coating gas from the container and the step of exposing the inner surface of the component which is to be coated may take place automatically under the action of the force of gravity, since the coating gas, which substantially comprises CrCl, has a higher density or weight than the gases of the atmosphere, such as the inert gas. Therefore, the heating continuously generates coating gas without further measures for generating or influencing the flow being required.
The process can be carried out in an inert environment, in which case, by way of example, Ar is used for purging purposes.
The component used may be a hollow turbine blade, the cavity of which serves to save weight or is for cooling purposes and the surface of which has to be protected against corrosion and/or sulfuration. The latter occurs in particular with hollow, uncooled gas turbine components.
To ensure reliable protection against corrosion and sulfuration, the diffusion layer is formed with a layer thickness in the region of 25 μm and a chromium content in the range from 17% to 20%.
Furthermore, according to the present invention an apparatus is provided having a container for accommodating a mixture of chromium granules and an activator, such as NH4Cl in powder form. At the bottom of the container, there is at least one outlet for a coating gas. A device for holding the component in such a way that the outlet of the container is positioned in the region of the inner surface of the component is also provided. It is possible for the apparatus to be arranged in a heatable retort for heating the mixture which is in the container to a temperature at which a coating gas which substantially comprises CrCl is formed.
To ensure advantageous dissipation of the coating gas, the base of the container may slope downwards towards the outlet or, for example, may also be of funnel-shaped design.
Furthermore, the shape of the outlet may be matched to the shape of a cavity in the component which has the inner surface, so as to ensure that the inner surface is completely exposed, without losses, to the coating gas.
To create an inert atmosphere, the retort may have a gas-feed device and a gas-discharge device for an inert gas, such as for example Ar, which is supplied and removed again for the purpose of purging the apparatus.
For relatively long coating times in the pulverulent activator, such as NH4Cl, is initially present in the mixture in an insufficient amount, the container may have a feed line 1b as shown in FIG. 1 for the activator which is in powder or in particular also gas form, through which line, for example, a mixture of HCl and Ar can be passed, with the result that further coating gas which substantially comprises CrCl is formed.
To improve the economic viability, a multiplicity of apparatuses may be arranged in the retort, in order to allow the simultaneous coating of a plurality of components. For this purpose, the apparatus may also have a plurality of outlets at the base.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.